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testing: Problems with memory allocation calculation for "json.Marshal" in Benchmarkc tests #68381

Description

@Fgaoxing

Go version

Windows10/x86_64 Go1.22.5

Output of go env in your module/workspace:

set GO111MODULE=on
set GOARCH=amd64
set GOBIN=C:\Users\User\go\bin
set GOCACHE=C:\Users\User\AppData\Local\go-build
set GOENV=C:\Users\User\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\User\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\User\go
set GOPRIVATE=
set GOPROXY=https://goproxy.cn,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.22.5
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=NUL
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=C:\Users\User\AppData\Local\Temp\go-build1301597658=/tmp/go-build -gno-record-gcc-switches

What did you do?

package main_test

import (
	"encoding/json"
	"fmt"
	"reflect"
	"testing"
)

func BenchmarkMain(b *testing.B) {
	for i := 0; i < b.N; i++ {
		jsonObj := struct {
			Test1 string `json:"test1"`
			Test2 string `json:"test2"`
			Test3 string `json:"test3"`
			Test4 string `json:"test4"`
		}{
			Test1: "test1",
			Test2: "test2",
			Test3: "test3",
			Test4: "test4",
		}
		Used(json.Marshal(&jsonObj))
	}
}

func Used(...any) {}

I also changed "encoding/json" to disable caching.

// Line:1269
// cachedTypeFields is like typeFields but uses a cache to avoid repeated work.
func cachedTypeFields(t reflect.Type) structFields {
	/*if f, ok := fieldCache.Load(t); ok {
		return f.(structFields)
	}
	f, _ := fieldCache.LoadOrStore(t, typeFields(t))*/
	return typeFields(t) //f.(structFields)
}

Normally func typeFields(t reflect.Type) structFields has a loop that iterates over the Fields of the structure, but this requires memory allocation, and the structure I created has 4 Fields, but he only allocates memory 2 times for this operation, which is unrealistic!

What did you see happen?

D:\Desktop\OAC\jsontest>go test -benchmem -bench=^Benchmark -v -cpuprofile=cpu.pprof
goos: windows
goarch: amd64
pkg: jsontest
cpu: Intel(R) Core(TM) i7-5500U CPU @ 2.40GHz
BenchmarkMain
BenchmarkMain-4          1899802               577.3 ns/op           144 B/op          2 allocs/op
PASS
ok      jsontest        1.777s

What did you expect to see?

D:\Desktop\OAC\jsontest>go test -benchmem -bench=^Benchmark -v -cpuprofile=cpu.pprof
goos: windows
goarch: amd64
pkg: jsontest
cpu: Intel(R) Core(TM) i7-5500U CPU @ 2.40GHz
BenchmarkMain
BenchmarkMain-4          1899802               577.3 ns/op           144 B/op          6 allocs/op
PASS
ok      jsontest        1.777s

Activity

  1. changed the title [-]import/testing: Problems with memory allocation calculation for "json.Marshal" in Benchmarkc tests[/-] [+]testing: Problems with memory allocation calculation for "json.Marshal" in Benchmarkc tests[/+] on Jul 11, 2024
  2. seankhliao commented on Jul 11, 2024

    @seankhliao
    Member

    I don't see why allocation needs to scale with the number of fields for marshaling, the only memory returned is the json byte slice.

  3. locked and limited conversation to collaborators on Jul 11, 2025
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